Receiver system



July 4, 1950 M. G. cRosBY 2,513,786

RECEIVER SYSTEM Filed Feb. 14, 1345 s sheets-sheet 1 July 4, 1950 M. G. cRosBY 2,513,786

RECEIVER SYSTEM Filed Feb, 14, 1945 3 Sheets-Sheet 2 Afro/Mfr.

JUIY'4, 1950 M. G. CROSBY 2,513,786

RECEIVER SYSTEM Filed Feb. 14, 1945 Tlc' .3. To @IiP/ff? F475? 3 Sheets-Sheet 3 INVENToR. Maf/efr 61 @may Patented `uy 4, i956 UNITED PTEN-r csr-ice v RECEIVER SYSTEM y' Murray G. Crosby, Riverhea'N. Y., assig'fnoi to Radio Corporation cf'America, a corporation f Applicatiniebmary 14, 1945, serial No. 577,902

My present invention relates tol amplitude modul-ated (AM) or phase modulated (PM) carf rier wave receiver systems, land more 'particularly to a system for receiving or PM -carrier waves with carrier exaltation'.4

Carrierexalted Ireception should come intov more and more general use in all wavel bands in which multi-path fading distortion is encountered. This includes the present AM broadcast band of 550 to 1600 kilocycles (kc.) in which fading distortion is experiencedin a. region of about 50 to 100 miles from the transmitter due to interference between the sky Wave .and theground wave. Such fading causes a cancellation ofthe carrier frequency, leaving themodulation sideband components to beat together thereby causing unintelligible distortion. Such distortion has been termed selective fading in the past, yand it is a severe type of disortion. A This form :of dise i). tortion is responsible for the fadingvvallwhich cast; The multiplicity of pathsa're caused by unequal numbers of ricochets ofthe signal between the ionosphere andy thev earth. The distortion that results is the same type that is experienced in AMv broadcast reception, except that it is present at allV times instead'of just at night. Hence, carrier-exalted'reception is more important in this band.

Having once applied carrier exaltation to an AM receiver, it is relatively simplefto convert the receiver to a PM receiver. This can be done by inserting `a 90 phase shifter inthe filtered car,-` rier circuit. Phase modulation then becomes the most economical transmission to use becauseof its advantages at the transmiter. liorexample,A

carrier-exalted PM reception would be advantageous in plane-to-ground communication where the band from 1.5 to 20'mc. is used, and Where the decreased Weight of the PM transmitter would be welcomed. VIn addition, the* elimina,- tion of selective fading eifectswould be realized.

By the terms carrier-exalted re- Furthermore, it has been found that in the pres:- ence 'of' strong noise. carrier-exalted reception generally' gives a slightly in'rprovedv reception.. This i'nprovement is apparently due to the el'irnf ination of beats between' the individualnois'e components, the beats presu'maltvly. being eliminated by Vthe, carrier exaltation. Carrier-exalted reception gives anY improvedy signalto noiseratio in thep'resence of strong man-made noise 'ofthe impulse type.` In the casefof this latter type of noise, carriier-exalted. reception appears to providean'elimination of low'frequency noise com p'onents which improves the" over-al1 character of reception. Q vIt is' onelof the important objects of my pres-I ent invention vto provide ra system 'for receiving AMor PM carrier waves in such a mannerthat the effects of selective carrier'fading are jvry greatly reduced, 4Tlfiis'is generally accomplished by`fclividing'the rceivedsi'gnals into two'fprathsf, filtering and limitingthe carrer in one path, and recombining'in a'pr'edetermined manner the limy'f ited "filtered signal with' the unlteredv signal at the detector. v

'Anotherimportant object of this invention'is to provide asuperheterodyne 'receiving system,

adapted to receive veither AM or PM carrier waves, wherein anv automatic 'frequency 'control' (AFC) system havingga high degree of control functions to maintainjthe intermediate frequency' (1." E.) value 'at a predetermined center frequency 'equal to the crystal frequency of a discriminatoire bcdyinga crystal. y

Another important object of' myl present "m5 vention is to provide a novelautomatic frequency control circuit fora superheterodyne radio're#l ceiving'system',' thefreq-uency control circuitfjen ploying b'othrelatively fine and rough controls over the` local oscillator network ofthe System and'thes'ource of AFC voltage'being a discrim'i nator which is characterized by `a relatively steep frequency response characteristic. y l Another object of -my invention isto` provide an' audio frequency-responsive burst control net'- work which'functions tol decrease the 'gainfof'fa signalampliertube prior ltoA the demodulator of the system whenever the audio frequencyY output volume bursts above a predeterminedintensity level.

Still another object of my invention is to p'rol vide a novel `form of AFC circuit fora radio receiving system, wherein the frequency-adjusting element is responsive to a comparison between relatively `low frequency currents, the corinparisor'i` being regulated in accordance wth'the generated .ml Y AFC voltage, and being further employed las a novel form f tuning detector device for the purpose of visually indicating the AFGperformance.

Yet another object of my invention is to provide a carrier-exalted receiver of AM or PM waves which includes a crystal filter network for providing filtered carrier energy for the carrier eX- altation function, the filtering, additionally,'pro viding a relatively sharp AFC voltage, an AFC squelch circuit being employed to respond to a predetermined decrease of carrier amplitude below a desired intensity level.

Still other objects of my present inventiouare to improve generally the efficiency, reliability and stability of receiving Systems adapted'to receive signals normally subject to selective carrier fading, and to provide such systems in a manner such that they may be manufactured and as sembled in an economical manner.

Still other features `and objects will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically circuit organizations whereby my vinvention may be carried into effect.

In the drawings:

Fig.. 1 shows a receiving vsystem for AM and PM waves embodying my invention; i f Figs. 2". 3 and 4 respectively show dierent ,modifications of the lburst control circuit.

Referring now to the accompanying drawings,

thereis shown in Fig. l a preferred embodiment ofja radio receiving'system employing my vpres'- ent invention, the numeral l designating a signal collector device particularly .adapted Vto collect signals in a range of 1.5 Vto 30 mc. The signal collector is specifically. shown as being a dipole,

but it is to be clearly 4understood that any other signal feeding device; or source may be utilized.

For example, a grounded antennal circuit, a radio frequency distribution line, or .any other suitable signal' supply source maybe utilized, It is tobe understood' that the'signals collected by the vdipole l may be either AM wal/.es or PM waves. The signals may, also, be frequency modulation (FM.) waves of a relatively restricted lfbreguency deviation range, since in that, case the signal character is closely akin, to the, character of, PM waves. Those skilled in the art of radio reeptionuare fully aware of the factjthat in .th'e

vcase of AM wave' transmission the carrier at the transmitter is` varied in amplitude, while maintaining the carrier frequency ccnstar'1-t, in respense to the variations of a modulationsignal.

wave transmission occurs in the range of1.5

to"30 mc. on various channels, andv in foreign broadcast reception such AM transmission is commonly employed yatthepresent'tiine..

l During PM transmission. theamplitudeof the carrier is maintained constant, While the phase ofthe carrier isL variedinlresponseto the modulation signal. 'Ihe extentofphase deviationor shift depends ,upon the amplitude -of the, modulation signal, such as audio frequencyamplitude,

whilev the Vrate .of .phasedeviationisa function of the modulation frequencies per se. In thecase ofnarrow band FMtransmission thaextent of Y frequency deviation is relatively restricted, and,

therefore, this type of transmission closelyvresembles PM transmission.. t

Regardlessy of the .character .ofv transmission thefproblem of selective carrierfadingds aserivsystern disclosed in this application provides a solution of this problem, which solution embodies devices adapted to cope with various factors which arise during selective carrier fading. It is tobe clearly understood that my invention is not limited to the specific frequency ranges mentioned above.

The tunable high, or radio, frequency amplier 2 is preferably .coupled to the signal collecous one, since there. occurs Vva Vformof distortion upon signal. reception which often renders fthe ,rlioduced Output unintelligible?. leghe-@Senf tor device l by va' switch 3. This switch is shown closed thereby indicating that signal reception is to be had in the 1.5 to 30 mc. range. The amplilierl 2 may consist of one or more adjustably` tuned .stages for selectively .amplifying the signals of a. desired transmission channel. The following cascaded stages 4 to 1 inclusive are all conventional in construction, and are, therefore, schematically represented.

The network '4 is a converter vnetwork which may be constructedV in anysuitable manner to convert the selected high vfrequency signal waves to a lower or I. F. value of,forv example, 450 kc. Any desired form of combined local oscillatorfirst detector circuit may be utilized for, this purpose. The carrierl frequency of VVVthe converted signal energy being reduced to 450 kc., the. I. F. amplifier 5; which may include'one vor more selective amplifier stages, functions toamplify the I, F; energy and applies it to the following mixer stage ii. The Ymixer stage, as is well known to those skilled fin theart, is supplied with 450 kc. signal energy `and locally-produced oscillations from vany suitable local oscillator. Since it is desired toprovidea secondI. F. value of.50 kc., the local. oscillations will have a frequency ofY 500 kc. and are supplied to the' mixer 6 by lead; B. The local'v oscillator network, which may be yof any vsuitable construction, is fixedl'y tuned by means of the tank circuit to the 500 kc. frequency. VThe mixer 6m`ay,.if desired, be providedv by a pentagrid tube having separatev grids to which the I. F'..energy andthe local Voscillation energy are` supplied.

I, utilize for the localoscill'ajtor a two-terminal oscillator circuit'ofthe'type disclosed'and claimed by 4.mein 1I. S.v Patent No. v2,269,2117 -granted` J anuary 6, 1942.' Since the specific constructionof the. oscillator circuit isno'ta part of my present invention, and since it' isfully described in the lastl ,mentioned patent, algeneral description thereoffis believedfsufjcient for the purpose of the. present.application.v The oscillator tube. l!) is.of the twin triodetype ,The cathodes Il rvand 12.of thetriodes are connectedin commonto ground through a resistorV i3. The tank circuit 9, tunedtd the operating, local` oscillation frequency of 500 kc., is connected between the. plate lli andthe 1 groundedendof .common resistor I3, thedirect current blocking`.condenser |,5.con`

Lend ofcathode resistor. I3.

The second.-r.con1 "trol `grid 2|` is connected onthe vone -handiby an intermediate tap. The upper section of the voltage divider 23 is bypassed byl condenser 23", and the local oscillatorenergy.- is tapped offfby lead 8 from the tap on divider :23. The numeral ..24 designatesa shielded transmission line, schematically represented-.throughwhich the leadS is to bev understood as being run to' the mixer 6. IAsvistated previously the tanklcircuit 9 is xedly tuned to 500 .kc.,land, of course, theucoil 9 :and condenser 9"4 of the tank .circuit .will have. their 4constants sochosen as to provide a normalres- -onant frequency of 500 kc. yThe mannerinwhich the frequency of tank circuit .9 may vb'exadjusted -With a relatively fine and/or rough adjustment 'will be described at alater point'.v 1 f' '.The output of the mixer stagewill beat the rlower or secondA value of 50 kc. `The amplifier 1 may includeone or' more stages of selec- IJtive amplincation forE the signals whose carrier ffrequen'cy has been reduced to lower or second 1. value of 50 kc. vThe output of the amplifier "1 is then utilized to provide the various voltages for performing the-functions lconstituting the'ob'- 'jects of my invention. v'l'.t isto be yunderstood that the successive `steps'of selective conversion and amplification ofthe signal are the same regardless of the nature of the signal received.- i

For example, vand assuming-switch 3 has been opened, the signals received by antenna I,after selective ampli'lication by amplifier network-2 at the received-carrier frequencyfmay be applied to the input terminals of 'the vconverter` 4. A 'switch 9' isl schematically representedas being lincircuit with the output amplifier 2^ so as to indicate that signal waves in thelower AM-fre quenoy band of '550 to 1600 kc'. 'may be utilized in theg present receiving systemshouldfit be desired to minimize selective carrier-fading Yeffects'. f

voltage may be applied to onejor more of stages i 12,4, 5,6 and 1 -depending'upon the wishes of the system designer. Those 'skilled in the artare fully acquainted with theI manner` of constructing anAVC circuit, and it is believed suiiicient schematically to indicatethat the LAVC line '26 fis connected to the network 2 Asoasi-to control the `gain thereof in a sense such as to compensate-for carrier amplitude variation. The second AVC line .25 is shown connected from the amplifier 12'-` to a second contact of -`switch' 21. The switch 21 may be adjusted to apply-the output of the AVC rectifier V25 to either` of the AVC lines 26 or 2S. The functions of lthe AVC 'connections arel Well-known; they act .to vary the gain of the respective signal' amplifiers in a sense .to compensate foracarrier amplitude variation. The signalv output ofthe 50 kc. amplifierA 1-Vis applied .through separate paths to' auxiliary '-50 kc. ampliiiers 28'an'd vi9irespectively. T-he shield-.- ing: means for transmissiony line 136|.v isfdenoted by .n The carrier filter and AFC discriminator, :will now bedescribed, generally follows the teachnumeral 3l`, and is schematically.represented. The line 30 terminates in an adjustableswitch -arm 32 which may be shifted from a PM` contactr t0 an AM contact. Between these contacts there is-connected a phase shifter consisting` of Aeither condenser C or resistor R in series with condenser 35, inductance31 and potentiometer 38. The constants of the phase shifter are so relatively chosen that when arm 32 is connected to the PM contact, there occurs a 90 phase shift inthe signal yenergy applied to signal grid`33 of amplifier tube 28. When switch arm 32 is adjusted to contact AM there willl be no shift in phase of the signal'energy. Itis desired to adjust arm 32 in accordance with the indi'- `cated reception of PM or AM signals. l

The cathode 33 of ampliiier tube 29 is connected to ground by resistor 34. The signal grid 33 is connected to ground by part of the phase shifter comprising condenser 35, iron core coil 31 and resistor 36 in a closed series circuit. An

adjustable tap 38, slidable along resistor'l," is

connectedto thegrounded end of cathode resi'sf'- Vtor y3 4. The potentiometer 36, 38 provides a range of phase shift adjustment which, when once set, gives the proper zero and 90o carrier phase relations when switch 32 is on contacts""Al\/I-and PM, respectively." The cathode resistor A34 provides self-bias for the grid of tube 28. "For simplicity resistor 34 is unbypassed so that'a small amount of degeneration takes place. plate or output electrode 39 of tube 28 includes in circuit therewith the resonant primary `circuit 40 of iron core transformer 4l. rThe resonant secondary circuit 42 of transformer 4l Uis' cou.- pled between the input electrodes of the followi- -ing 50 kc.'amplier tube 43.

. The plates 39 and 44 of amplifier tubes 26 and 4 3 respectively are supplied in common from the +B terminal. Filter resistors 45 and 45' are respectively included in circuit with the primary .windings 4I. and `4t of iron core transformers 4| and 46 The respective screen grids of'tubes 28 and 43 are supplied with suitable positive po. ,tentials by'virtue of respective screen resistors `41 and 41'. of suitable value and properly bypassed at either end thereof by condensers.v EachI of tuned circuits 40, 42 and 48 is sharply selective to kc.j v which ings of my application Serial No. 536,093, filed May 18, 1944, now Patent No. 2,416,911, dated March 4, 1947. this network includes a piezo-electric crystal 49 which Vis tuned to the operating l. E'. `value, which in this case is 50 kc. The metallic input v*electrode 49 of the crystal lter 49 is connected to one sideof the secondary Winding 45 of iron core Vtransformer 46. rlhe opposite end of winchv ing46 is connected to the metallic. output. elec.-

' trode 43 of the crystal 49 through a condenser 50 which functions las a device for neutralizing the capacity between the crystal electrodes. Con- Theisecond path consists of resistor 51 which is shuntedvby condenser 58. It will, thereforajbe seen rthat. the relatively pure carrier energyr out'- The discriminator section ofr sists of a pair of triodes whose cathodes12 and 13 are connected in common to ground bythe unbypassed resistor 14.y The control grid is connected to ground through the resistor 15, and the direct current blocking condenser 16 is inserted in the path from lead 66 to grid 10. The plate 11 of the input triode of tube 1| is connected -to the +B terminal of the direct current source through a path which includes a carrier on-off switch 18, resistor 19, lead 8|) and -a carrier strength indicator 8|. The indicator 8| may be a milliammeter, or any other suitable device, for indicating the intensity of the carrier energy! delivered to the subsequent demodulator.

The output triode of tube 1| includes the `co trol grid 82 which is connected to ground. The plate 83 of the output triode is connected to the upper end of coil 84 of the iron core transformer 85. The lower end of coil 94 is connected to grid' 82 by the bypass condenser 86. Coil 84, which is the primary winding of transformer 85, is tuned by shunt condenser 81 to the operating I. F. value of kc. It will be seen that the plates 83V and 11 are each positively biased from the +B terminal, and that switch 18 functions to controlv the injection of filtered carrier energy into the' demodulator circuit. If the switch 18 is open, then' the limiter circuit is rendered inoperative thereby preventing the application of the filtered carrier energy to the dernodulator circuit.

-It is not believed necessary to describe the functioning of the limiter 1| in any detailjsince the description of its operation will be found in my aforesaid U. S. Patent No. 2,275,565. It is` sufcient for the purposes of this application to point out that when grid 10 is made positive increased cathode current is drawn through cathode resistor 14. Hence, the lcathode's 12 andy 13 are made more positive with respect to ground. Malz,k ing the cathode 12 more positive with respect to ground is equivalent to making grid 82 more negative. Thus, a positive change on the grid 10 effects a resultant negative change on the grid 82. This phase reversal causes the output triode to effect the negative grid limiting for the positive half cycles of the input wave, while .the input triode effects the limiting for the negative half cycles. Thus, when grid 10 is swung negative, negative grid cut-offY limits the change in cathode current caused by the input wave. When the grid 18 is swung` positive, then grid 82 is effectively swung negative until negative cut-off is reached for the output triode.

The limited carrier wave energy is appliedV in parallel to the opposed diodes 88 and 89. The secondary winding 9D of iron core transformer 85 is connected between the junction of load resistors 9| and 92 and the midpoint 93 of secondary coil 94 of the iron core transformer 95. The coilv 94 `is shunted by condenser 96 which tunes `the secondary circuit to the operating I. F. Value of 50 kc. The primary coil 91 is shunted by e011-, denser- 98, and the latter tunes the primary Icircuit to the same frequency value. The entire transformer' network. provides a band `pass response curve l0 kc. wide at 50 kc. The primary circuit 98, 91 and secondary circuit 94, 96 are shunted by respective damping resistors 99 and to insure a flat top for the band pass response curve of transformer 95. Load resistors 9| and 92 are each shunted by respective carrier bypass condensers. `The cathode end of resistor 92 is grounded, and audio frequency voltage is taken off from the cathode end of load resistor 9i.

-'Ih'e modulated carrier waves, reducd'to vthe 50 kc. frequency, are applied to the yprimary circuit of transformer 95 by'means of the amplifier tube 29. The signal control grid ofthe latteris connected tothe slider .|03'of potentiometer resistor |04 Whoseupperfend is connected by di1l rect current blocking condenser'l and lead |06" to the lead 3|). Thelowe'rend of potentiometer resistor |04 is established at ground for 'signal' frequency potentials by'the condenser |91." Ad'-1 justment of slider y'|93 determines the intensity of the signal energy'a'p'plied to grid |99;` Cathode- IUI of tube 29`is` connected tov ground by unbypassed resistor |02.' The plate circuit of amplis! fier 29 includes a signal on-oif switch |98, which" acts to connect or disconnect the-l-B terrninaly from theplate |0901? the tube'. In' general, the: amplier 29 may be constructed in a manner sim-'f' ilar to the I. F. amplifier 43. i It will-be seen thattheamplier' tube 29 "ap-I plies the 'amplified signal voltages in push-pull; or phase-opposed, relation to the pair of diodes. This follows from the fact" that the anode of vdiode 88 is connected to one end ofcoil 94,-whil'e thel anode of diode 89 isl connected 'to the opposite e'nd' of coil 94. The midpoint93is at ground potential for I. F. currents by'virtue'of themidpoint beingAT connected to ground through a path comprising."y lead 93', coil 90, lead 90' and condenser 92'.2 For PM signal detection the ltered 50 kc. 'carrier energy is adjusted to'be .in phase quadrature with the unfiltered, amplifledsignal energy at the re spective diode anodesby means of phase shifter 35, 36, 31, and C. Asthe phase of the signal' en ergy at transformer 95"'-rapidly deviates relative".y to the phase quadraturev position, unequal signal. voltages are applied to thediodesthereby topro-" duce unequal rectified voltages across the load reif sistors 9| and 92. "The differential'of the rectified voltages corresponds to them'odulatiomor audio; signals to be reproduced; If the switch 32 isadjusted to the AM contact, then the filtered car-` rier energy is appliedto coi1i9' of the demodulait tor at a phase difference of zero or with th unlteredsignal carrier. In other words,` thef phase shifter now includes 35, 36,331 and R. Push-1 pull AM detection Will result, with `carrier exalta-vtion due to thev high level of the lteredlcarrierfati the common input circuit of the opposed'diodes: 88and89. f .I f f rv f The modulation signal voltage'may be 'amplifiedl in as many audio frequency amplifier 'stages as' i'si desired. The rst audio 'frequency amplifier -tube ||0 is shown as having its input-electrodes'adjustably coupled across vthe demodulator output? load by thepo'tentiometer yThe controlv grid:

I l2 is `connected to the cathodeend of load 're, sistor 9| through ay path including resistor IIS5 and condenser. I4 in series. The lower'end offre-fi sistor |=|`3 -is connected to ground through'atrap 601i circuit* consisting of a series' tuned circuitreso-ff Varying the intensity of the audio frequency signals to be ampliiied. The -plate circuit of the? tube ||0 includes the primary winding of theau dio frequency output transformer.- I |`1, andf'it will? be understood that theI transformer |1 may'feedfil' any desired type of audio-frequency output'.nei'-.f-L

other of such condensers. In this way there is produced increased energy on one of rectiers 53 and 54 and decreased energy on the other,

solas to create a direct current potential across rectifier load resistors 63 and 64. y A4Upon change of the incoming frequency in theopposite direction, the phase of thelteredoutput of the crystal changes in the opposite direction thereby decreasing the energy on the rectifier. where it before was increased, and increasing the energy on the other rectifier so as to produce a reversed direct current potential drop across the load resistors. quency discrimination action is similar in certain of its aspects to that described in my application Serial No. 484,973, filed April 29, 1943, now Patent No. 2,363,652, dated November 28, 1944. The AFC discrimination action is the same for AM and PM signal waves. Obviously, the direction and extent of variation of the reactive impedance of the reactance tube |23 will depend upon the variation in AFC bias transmitted over lead $1. This, of course, is the wellknown AFC action which is familiar to those skilled in the radio communication art. In this case' the AFC circuit functions as a, system having va high degree'of control to maintain the mean frequency of the signal energy applied to circuit 48 s-ubstantially equal to the predetermined reference frequency, which is the resonant frequency of the piezo-electric crystal 49, for the best operation of the circuit.

In accordance with another feature of my invention, an improved motor-driven, or rough adjustment, AFC circuit is provided for the local When the slow drift of the oscilla"` oscillator. tor has caused the effective frequency value of the I. F. energy to drift to a point beyond the the stator |35 thereof connected by lead |36 to thehigh potential side of tank circuit 9. The adjustable or variable element |31 of the con denser is effectively at ground potential by virtue of the contact between the grounded metallic housing |38 and the metallic driven shaft |39. Hence, the capacity of condenser |32, which may have a, normal value of about 25y micromicrofarads (mmf), is electrically shunted across tank circuit 9. In other words, condenser |32 supplements the electronic con.- denser provided by the reactance tube The adjustable element or rotor |31 is yactuated. by .a two phase motor which is schematically represented. The motor may be of any suitable construction. 'I'he system employed for energizing the two phase motor is generally disclosed inv my-application Serial No. 393,339, led May 14, 1941, now Patent No. 2,380,947, dated August 7, 1945, especially Fig. 1 thereof. f l

This fre- In addition to the shunting of a motor-2 :The numeral |40 designates the rotor of the,

two phase motor. `The latter is schematically.

represented, since its construction is well lknown in the electrical art. The energizing windings of` the motor rotor |40 are represented by numerals |4| and |42. These windings are wound on a.

common iron core, and each carries 60 cycle cur-` rent derived from the 60 cycle supply source. The source of 60 cycle current may be the usual power line source of alternating current at 110,

The numeral |43 designates the source of volts.. 60 cycle current, and vit will be understood that from sourcev |43.

opposite end kof winding |42 through condenser |46 and lead |41 to the second contact of switch |45. When switch |45 is opened then Winding |42 is cle-energized, and

" the twophase motor cannot be operated. Hence,

1 tuation yof gear |49.

element |37 of variable condenser |32, This mechanical coupling may be provided in any suitI able manner, but is schematically illustrated herein as being provided vby a shaft |48 which carries a drivinggear 49. The rotor shaft |39 is providedwith a section |39 to which it is coupled by. means of a telescopic coupling. It is not necessary to show the mechanical details of the telescopic coupling; Since those skilled in the art of constructing mechanical tuning elements are well,` acquainted with the construction .of such a coupling., The numeral |50 schematically represents the telescopic coupling, and it is to be under-` stood that the latter may be generally of the type wherein' the adjacent ends of sections |39 and |39' are telescoped.

For example, the end of section |39 may be`v provided with a keyy which is norm-ally in engagement with a slot in the end of section |39' so that the Atwo sections then may be variably telescoped to -allow gears |49, |5| to be disengaged. This would'permit adjustment of rotor l'lwithout acsuch action so'as to permit the driven gear |5| of shaft' sectionv |39' to be disengaged from gear |49 Vin response to the pushing of the knob |52 against theA bias of spring |53 in an axial direction. In other words, when it is desired to change the normal adjustment of rotor |38 it is merely necessary to push knob |52 (which may be mountedon the exterior of the operating panel and atthe upper end of shaft section |99) in a direction :towards condenserA |32. This will cause the end of;shaft section |39 to move axially relative to'- the end of section |39, and thereby permit gears i |49and |5| to become disengaged. The knob |52 may thenl be rotated for adjusting rotor |31 to any suitable point so as to provide a desired nory mal capacity across tank circuit 9. Upon release ofknob |52 spring |53 will push the knob into normal position, and permit gears |49 and |5| to engage In explaining the operation of the two phase motor it is first pointed out that cycle current kmust flow through windings |4| andk |42 in order to actuate the motor armature, or rotor, |40.- Furthermore, these currents must be in` :phase-quadrature in order to energize the rotor .y

t is desirable to provide 15 |40. The 60 cycle current owing through winding |42 is derived directly7 from the 60 cycle source, but the phase shifter condenser |46 is sovr chosen in magnitude as to provide a 90 phasev in vwinding |4 If the AFC bias transmitted over lead 61 is zero, there will be no flow of 60 cycle current through winding 4|. However, if AFC voltage is developed, such voltage will cause 60 cyclercurrent to appear in winding |4l, and the current willhave a magnitude and direction depending respectively upon the extent and direction of the shift of the mean` frequency of the appliedl modulated carrier energy from the predetermined frequency of circuit 48. In this way there is provided a method of operating the two-` phase motor at variable speed and in relative directions. This permits an accurate control over the adjustment of rotor |31. y K

The control circuit for the 60 cycle current in winding |51 involves the following circuit ele-'- ments. The tube |54, which may be a pentagrid tube of the 6SA7 type for example, has 60 cycle voltages applied to spaced control grids |555 and |55 thereof. AThe cathode of tubey |54 is connected to ground through a grid bias resistor |51 of about 1500 ohms. The rst grid |55 is connected throughdirect current blocking condenser' ||"('abo`ut 0.1 mf.) to the ungrounded end of potentiometer resistor |59. The latter has avalue of`about 20,000 ohms, and its slider |60 is connected to the third grid |55. The plate |51v and screen grids |62 are connected through respective resistors |53 and |55 (500,000 and 150,000 ohms respectively) to lead |65 terminating at the +2501y volts power supplyterminal.

The ungrounded end ofresistor |159 is connect? edi by lead |55 and' resistor |61 to oneend ofthe secondary winding'- |i38 of 60 cycle transformenv 50- The primary winding is shunted across the' source terminals |43, and includes resistor |1| |69.l The center tap of winding |68 is grounded.

(about 8000 ohms) in circuit therewith. Hence, potentiometer resistor |59 is effectively shunted across half of winding |58, the resistor |61 (of about 3 megohms) being in series with resistor' |59". Adjustment of slider |60 results in the 'ap-y plication of 60 cycle voltages of differentphases" to spaced grids and |55. The slider |60 is so adjustedon resistor |59 that in the absence of AFC bias on grid |55 the alternating voltages baly ance out in the plate circuit of tube |54. .y

Thisbalance of voltage in the plate circuit is |55v has two phase reversals, while that from gridv |55 has only one. Hence, when voltages of the same'phase lare fed to the two grids they oppose,v

Hence, in orderl to energize the two-i.

and 'cancel at the pia-t. The relative' magnitudesl of the 'voltages' fed to the two grids vare adjustedr `by slider ii'on potentiometer resistor |59. The

' position, AFC bias is applied to grid |55 from leadr adjustment is such that when switch |14 is in the off position (grid |55 grounded), the voltages fed to grids' |55 and |56 balance in the plate circuit. When switchl |14 is thrown to the "onf 61. The control voltage is fed to'grid V| 55 through grid resistor |12 'and time-constant circuit |13,

Depending on the polarity and magnitude of' f theAFC bias applied over lead 51, and assuming in a negative polarity sense, and assuming th e 60 cycle voltage Yon that grid isnormally of a negative phase, vthen the relatively positive 60 cycle voltage on grid |56 will 'cause the appearance acrossplate resistor |53 of 60 cycle voltage of relatively positive phase.`- 0n the other hand,

voltage across resistor'A |53 of relatively negative phase. Tit will', therefore, be seen that the AFCY 'Gexample be a pentodej of the 6AG'1 type. The

control grid |11 of tube |16 is coupled by con-4 denser' |13 to the plate end of resistor' |63. The

controlrgridy |11 is returned to the grounded endv of the grid' bias resistor |13 through resistor |80. The plate |8| of tube |15 is connected to the positive voltage'supply lead |55 through windingV V- bypassed to ground for alternating currents by condenser |83. Winding |4|` is shunted' by con'- denser ,|84 (of about 0.75 mf), and functionsto tuneA the winding Mi to resonance with the 60' cycle,` current desired for'energizing the motor.

It will therefore, be seen that the two-phase motor when energized will varythe angular position of rotor |31 in a sense to provide correctionat the tank circuit 9. When knob the rotor |31 isdisengaged from the motor. Rotation of the lrnob, while still depressed, turns the rotor |31',"and in this way condenser |32 may'l be centered so that it has an equal plus and minus range of control. It is to be understood that knob |52 will be provided with proper dial calibration.

'The AFC squelch circuit is desired to functionv nected to the tube |54 so as to shunt the platecircuit of 4| 54 vby the relativelyv low plate resistance of' the'two triodes in tube 200 when the grid end of resistor 'i5' assumes a negative potential less" than a predetermined` amplitude. This shunt reduces then amplification of tube |54 so that the 60 cycle voltage fed to tube |15 is insucientto:

operate motor |40. In this way 'it is possible to |52 is pushed against spring` |53, as stated previously,`

In such case the grid'.

rier becomes excessively low, or disappears, due

to fading. v

The AFC 'squelch tube 200 maybe ofl any suitable construction, although specicall'y represented as a twin-triode of the 6SN7-GT type, and has its'elective plate to cathode impedance shunted across the plate to cathode path of tube |54.

"Thus, the plates of both triode sections of tube 200 are connected together, and by lead 20| to plate |6| of tube |54. The control grids of the two ytriode sections are connected in common to lead |34. The latter (designated as AFC S quelch) is connected to the grid end of resistor through the resistor element of filter network |34', there- Yby to prevent the impression of any alternating voltage on the grids of squelch tube 200. The cathodes of tube 200 are jointly connected i through a squelch switch to ground. To be effective the squelch switch must be closed thereby providing a squelch on adjustment. It will be evident that tube 200 constitutes an electronic load on the plate circuit of tube |54. The electronic load varies in magnitude depending on the magnitude of the normally-negative voltage applied overflead |34 to the grids of tube 200. The negative voltage developed at resistor l5, duev to grid current drawn by the output triode section during normal operation of limiter tube 1|, is applied through time constant circuit |34', and functions to reduce the plate current of tube 200 to a minimum. Hence, there is minimum diversion of plate current in the plate circuit of tube |54, and the lattel` operates eiciently. However, when the carrier decreases below a usable amplitude, as during lbad fading, the negative voltage at resistor 'l5 decreases to a minimum value. This causes the plate current of tube 200 to rise to a maximum thereby causing sufficient shunting action `on tube |54 to cause the two-phase motor to cease operation. In this way the squelch circuit prevents undesired interfering signals from taking control of the motor-driven AFC during periods of excessively low carrier amplitude.

The condition of resonance of the receiver is visually indicated by tube |33. The latter is a cathode ray tube indicator of the typeproviding a pair of electronic shadows or non-illuminated areas. The tube |33 may be, for example, one of the GAFG-G type comprising an electron emitter 2|0, connected to the grounded center-tap on transformer secondary |68, a iluorescent' target electrode 2| Land a pair of electron ray control electrodes, o1' rods, 2|2 and 2|3. The rods 2 l2 and 2|3 are connected to the respective ends of winding |68, and are normally biased by the algebraic difference between the negative voltage drop across the cathode resistor 2|4 and the 92 volts across each half of the winding |68. The target electrode 2|| is connected byflead 2|5 to i the +250 volts terminal through winding |4| and between the cathode 2|0 and the outwardly flar- 7' ing target 2| The rods are vertical and parallel to cathode 2|0, but are located on respectively opposite sides thereof. Normally, the positively charged target 2||, whose innery conical face is coated Iwith a fluorescent material, attracts electrons `from the cathode. The electrons 4fall on the coated face of the conical target and cause illumination. If the control rods 2|2 and 2|3 are at minimum negative bias relative to cathode 2 0, then the rods have noveffect on the electrons. However, if therods become increasingly negative, they repel electrons and cause .a non-illuminated area, or electronic shadow to appear on the respective adjacent portions of the target. What is desired for indication of resonance is that the electronic shadows be of minimum angular width for in-tune condition of the receiver. However, for foi-tune to either side of the desired carrier frequency, it is desired to have one or the otherof the shadows to widen out thereby indicating both the sense and degree of off-resonance. In order to adjust the tube 33 to indicate balance condition, that is with both shadows of minimum equal widths, the slider is adjusted until that indication is provided. It is best rst to adjust switch |74 to 01T position, and observe the indication of tube |33. If tube |33 indicates balanced shadows, then no adjustment is needed for slider |60. However, if an unbalanced condition is indicated, the slider |60 is shifted until balanced minimum shadows are attained. In the balanced indication state, the target 2I| has a positive voltage determined solely by the +250 volts applied thereto. balanced condition, there is no alternating voltage appearing at plate |8|. Hence, the yrods12l2 and 2|3 are at minimum negative bias.

However, if 60 cycle current flows through winding |4| in one phase or the opposite phase,

the voltage of target' 2| 'l will vincrease thereby causing more space current to flow through resistor 2M. Thisy additional-space current will consistof an alternating current of aphase which will be'equalv to that on one of the ray-control electrodes, and opposite to that on .the other. AS a result, the alternating current biasformed on cathode resistor 2 4 will increase the effective bias of one ray-control electrode, and decrease that of the other. This relative difference in ray-control bias causes one of the shadow angles to increase,

and the other to decrease. It can be seen that as the phase ofthe voltagefon winding |4| is reversed by thev action rof the control voltage fed from lead 61, Ithe dissymrnetry of the shadow angles, also, reverses. Thetuning indicator thus amount. In Fig. 2 there is shown a burst control system which adjusts the output volume in accordance with a standard of reference, instead of merely reducing the volume peaks. The standard ofy reference is the detected output lvoltage of diode detector 300 whose input terminals are coupled by leads 30| to the crystal input circuit 48. It will be understood vthat the schematic representations 20, 53', 88' and ||0 include the corresponding circuits of Fig. 1.

The detected output of detector-300, the-latter having input terminals coupled to the I. F. amplifier output, tends to follow the normal volume variations ofthe transmitted signal. This would be especially true vfor diversity reception. The

This is true since for the kaudio.amplifier III), if desired. -double-diode tube 3I3, separate diode tubesy 'may .ing of transformer. SIU.

detected-output 'voltage vof the carrierexalted :demodlllator circuit88 (thefdetected, output, :of diodes =t-and 89-in"Fig. 1) is, however, -applied through transformer 3I2 to a pair of diodes em- 1 bodied. in-tube.. 3I3. lThe transformer primary 3I2n1ay, of course, be coupled to thei-nput'of be used. The anodes 302l and 3&3 of the pair of diodes in tube3l3 are-connected to vrespective f `ends of secondary wind-ing 304. The midpoint of winding 304 `is .connected by lead 305 to the upper `end of loadl resistor l-lii AThe latter is connected series with load resistor 3i5 to ground, and

eachofzresistors 3M and '3HE is bypassed by respective condensers SIB and BI'I fory alternating currents. v.

. TheL cathodes 302(v and 303', .are connected-in commonby lead 300 tothe lower end of load resistor 3M. Resistor 3I5 acts as the load resistor f for` rectifier 3H. The latter is, also, shown as a vdouble diode rectifier whosecathodes are connected tothe ungrounded end of load resistor 3 I E5 by lead-30ii. Thea-nodes ofrectiiier 3II are connected .to .opposite ends of thev secondary wind- The center-tap on the secondary .winding is groundecLand;v therefore,

Ithe detected output of detector 300 will be Arecti- -fied by rectifier 3I'I to provide a Voltage across resistor 3I5 proportional to Volume variations of the detected signals.

The differential yvoltage of the rectified voltages across resistors 3M and 3I5 is applied over lead |20 lto the signal control gridof I. F. amplinerL 29 which amplifies the unltered signal fed to the carrier-exalted demodulator y188. It will .bef noted that the rectiied voltage appearing across resistor 3I5 is a positivereference voltage.

The negtaive rectified voltage developed across resistor 3M must equal this positive reference In place ofa` voltage, or a control potential will be lapplied to the controlled I. F. amplifier 29. The control potential will be positive if the output of the carrier-exalted demodulator 88 is less than the detected output of detector 300. The latter `Would be the case for a volume dip in the l carrier-exalted output of deinodulator 88'.

l5a positive control potential (that is Where the voltage across resistor `SI5 exceeds the voltage across 3M) is applied over lead I20, there is caused an increase ink the .gain of controlled amplifier 29 so as to bring the amplitude ofthe i output of demodulator 88' up to equal that of the output of detector 300.

The control voltage applied overl lead I2() is of negative polarity when the rectified voltage 1 across resistor 3M exceeds the rectified voltage across resistor 3 I 5. This occurs when the output of the demodulator 88 is greater than the output lof detector 300, as would be the case for a volume burst. The control of the amplifier 29 thus 'has the effect of applying an automatic Volume control tol the output of demodulator 88 in a man- "ner such as to maintain equality between the detected outputs of the carrier-exalted demodula'tor (and the detector 306. Since 'the output of the diode detector 300 more nearly approaches the volume variation of the or-iginal'trans'mitted signal, an improved volume range is obtained.

The controlled amplifier-29 in Fig. 2 amplies "the Auio'iiltere'dsignal`energy which' is fed tothe "carrier-exalted demodulator. 'of'controlpotential to an amplifier which amplies 'the 'modulated carrier wave energy, f as `is true fofl'amplierfZS, has the advantage that lessy dis- The application 4controlled audio frequency amplifier I I0'. leadi306 would be connected in the same manner tortionfisproduced by the required `variations.irl

gain. gIf the .control potentialwere applied ,to the detected, voltage, f there is therpossibility :of introducing harmonic distortion due to -the requiredvariation of bias. This harmonic distortion falls'in the audio range, and is'rharrnful. When the controlled amplier amplies the modulated wave, theV harmonic distortion -thats produced is a harmonic of the carrier frequency ofthe modulated wave. .This harmonic is removed by frequencyselection. The result is not .,a. complete 'elimination of the possibility of distortion, but a reduced possibility of production.

In Fig. 3 I have shown how the jcontrol potential,which` is'the diiferential voltage of the rectied voltages v,across resistors sld. and SI5, would beapplied to a controlled ampliiier which functions to amplifyfthe outputof the carrier-exalted detectorv'.v It will be noted that thevcircuits of Fig. yS are substantially the same as those ofl Fig. 2except for the fact that the differential voltage of the rectifiers 3H and 3I3 is applied overlead SIS to the audio frequency amplifier III), The audio frequency amplifier II 0 is .pro-

lvided with an` input plug 320' adapted to bein- Aserted into the output terminals SIS of the carrier-exalted ,I detector 88'.. i

vfed to a controlled amplifier IIO which is itself not in the circuit of the voltage fed to; the rectiyfiers.` This-modification is different from that Vof. Fig. 3 in that thev control is not in the form of a feedback `control voltage, but isfrather in the form of acontrol on a separate amplifier which f is lindependent of the voltage fedto the rectiers. In other words, the lead. 305 to the resistor 3M would be connected to-the center tap of the Ysecondary winding ofy an audio` "transformer whoseprimary winding would be fed froma point in the audio frequency output circuit of` carrier-exalted demodulator 88 ahead of the The as shown in Fig. 3. When the control is applied in the Amanner vshown 'in Figufl, the decrease of control may be carried to such an extent that over-control is obtained. This high ydegree of `control may not *be` obtained by the circuits using the feedback control, but the feedback control circuits maybe somewhat :more stable. Y

WhileI have indica-ted and described several systems for carrying my invention into -effectjit will beapparent to one skilled in the 'art that myI invention is byno4v means limited to the-'particular circuit organizations shown and described, 'but that many modiiications .may be made without departing from the scope of my y invention. A

What Ifclaim is: Ina carrier-exalted receivingsystem., means =for amplifying collected modulated carrier waves `sub'jectwto' selective fading,l selective means for removing substantially all ,of the modulation :fromY the carrier waves to provide substantially 75pure :carrier energy, digscrim-inator -m'eans v,cou-

pled to the output of said selective means and to the output of said amplifying means to provide a frequency control bias in response to a shift in carrier frequency from a predetermined reference frequency and Within a predetermined range, means coupled to the output of said selective means for limiting the amplitude of said pure carrier energy, a demodulator for combining the amplified modulated carrier Waves with said limited carrier energy to providedemodulation of said modulated carrier waves, an electronic reactance device responsive to said frequency control bias for controlling the frequency of said amplified modulated carrier Waves, a twophase-motordriven condenser device for controlling the frequency of said amplified modulated Waves, a common source of alternatingr current supplying the two phases of motor-energizing current, said frequency control bias controlling the amplitude of one of said two phases of current, and means, responsive to a predetermined decrease of filtered carrier energy at said limiting means, for rendering at least one of said frequency controlling devices ineffective.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the file of this patent:

Number 22 UNITED STATES PATENTS Name Date Ohl May 26, 1936 Crosby July 12, 1938 Robinson Nov. 29, 1938 Biers Jan. 24, 1939 Gerth et a1. Sept. 26, 1939 Mountjoy et al May 7, 1940 l`Grroenveld May 7, 1940 Haffcke July 30, 1940 Herold Sept. 10, 1940 Foster Nov. 12, 1940 Braden Dec. 24, 1940 Jackson Feb. 11, 1941 Katzin Feb. 18, 1941 Beers Apr. 1, 1941 Travis Apr. 29, 1941 Kircher June 24, 1941 Beers July 29, 1941 Foster Feb. 17, 1942 Harris June 16, 1942 Hintz Aug. 25, 1942 Dome Nov. 9, 1943 Maynard Feb; 15, 1944 Mathews Nov. 27, 1945 Koch July 30, 1946 Ginzton et al. Nov. 12, 1946 

